Revista de la Sociedad Geológica de España 31 (1)1)/RSGE31(1)_p_89_104.pdf · 2018-06-14 · Revista de la Sociedad Geológica de España 31 (1) ISSN (versión impresa): 0214-2708

Revista de la Sociedad Geológica de España 31 (1)

ISSN (versión impresa): 0214-2708ISSN (Internet): 2255-1379

89

Revista de la Sociedad Geológica de España, 31(1), 2018

Abstract: An integrated sedimentological and paleontological analysis has been carried out in the lowermember (d2a) of the shallow-marine Nogueras Formation (Lower Devonian, Iberian Chains). This forma-tion represents the first carbonate-dominated and fossil-rich sedimentary unit of the Devonian of the IberianChains. Nine sedimentary facies, including terrigenous-clastic, mixed and carbonate facies, which are com-plexly intercalated at bed scale, have been characterized. Based on their sedimentary features and their la-teral relationships using Markov chain analysis, two sedimentary models for the lower and upper part ofd2a member have been proposed, which represent deposition in a mixed clastic-carbonate shallow marinedepositional system. They include terrigenous-clastic intertidal deposits and predominant skeletal, carbonate-dominated and grain-supported facies in the high-energy shallow subtidal zone, whith a clear zonation of theskeletal components (brachiopods, bryozoans and crinoids, from shallow to relatively deep areas). Phos-phate nodules, phosphatized fossils, ferruginous crusts and iron ooids, which are frequently associated withthe relatively shallower bioclastic brachiopod facies, were probably linked to mineral continental sources andto remobilization in the shallow water high-energy area. The paleontological analysis shows that some ofthose organisms lived in protected areas of the subtidal zone, including in particular high-diversity commu-nities of brachiopods, adapted to turbid waters with fine terrigenous suspended sediments.

Keywords: Lower Devonian, mixed clastic-carbonate platform, brachiopods, Iberian Chains, Santa Cruzde Nogueras, Teruel.

Resumen: Se ha realizado un análisis sedimentológico y paleontológico integrado del miembro inferior (d2a)de la Formación Nogueras, que representa la primera unidad marina somera predominantemente carbonatadadel Devónico de las Cadenas Ibéricas. Se han definido nueve facies sedimentarias terrígeno-clásticas, mixtasy carbonatadas, que están complejamente intercaladas a escala de capa, depositadas en un sistema mixto de-trítico-carbonatado de aguas someras. En función de sus rasgos sedimentarios y del análisis de sus relacioneslaterales mediante cadenas de Markov, se proponen dos modelos sedimentarios para la parte inferior y supe-rior del miembro estudiado. Los dos modelos incluyen depósitos terrígenos en la zona intermareal y facies car-bonatadas bioclásticas en la zona submareal somera, con una clara zonación de sus componentes esqueléticosdominantes (braquiópodos, briozoos, crinoides, desde la zona somera a la relativamente profunda). Los nódu-los de fosfato, fósiles fosfatizados, costras y ooides ferruginosas frecuentes en las facies bioclasticas de bra-quiópodos relativamente someras, se relacionaron probablemente con aportes minerales desde el continente yretrabajamiento en la zona marina de alta energía. El análisis paleontológico muestra que algunos de estos or-ganismos vivían en áreas protegidas de la zona submareal, incluyendo particularmente comunidades con altadiversidad de braquiópodos, adaptadas a aguas turbias con elevado sedimento terrígeno fino en suspensión.

Palabras clave: Devónico Inferior, plataforma mixta terrigeno-carbonatada, braquiópodos, CadenasIbéricas, Santa Cruz de Nogueras, Teruel.

Pérez-Pueyo, M., Bádenas, B., Villas, E., 2018. Sedimentology and paleontology of the lower memberof the Nogueras Fm (Lower Devonian) at Santa Cruz de Nogueras (Teruel, NE Spain). Revista de la So-ciedad Geológica de España, 31(1): 89-104.

SEDIMENTOLOGY AND PALEONTOLOGY OF THE LOWER MEMBEROF THE NOGUERAS FM (LOWER DEVONIAN) AT SANTA CRUZ DE

NOGUERAS (TERUEL, NE SPAIN)

Análisis sedimentológico y paleontológico del miembro inferior de la Fm. Nogueras (Devónico Inferior) en Santa Cruzde Nogueras (provincia de Teruel, NE de España)

Manuel Pérez-Pueyo, Beatriz Bádenas and Enrique Villas

Departamento de Ciencias de la Tierra. Universidad de Zaragoza, C/ Pedro Cerbuna, 12, 50009 [email protected], [email protected], [email protected]

Introduction

The shallow marine sedimentary successions of the UpperSilurian and Lower Devonian of the Iberian Chains (NE Spain)crop out with relative continuity in the Herrera Unit, in particu-lar in the so-called Depresión Axial del Río Cámaras (DARC;Teruel province; Fig. 1A-C). The DARC represents a key area ofthe Iberian Chains to analyze and correlate the Lower and Mid-dle Devonian stratigraphic successions of the Ibero-ArmoricanArc domain (Carls, 1999). The sedimentation of the Lower De-vonian successions of this area took place in a small intracrato-nic marine basin, the so-called Ibero-Armorican Trough (IAT),

located between the emerged areas of the Cantabro-EbroianMassif (CEM) and Central-Iberia (CI) in the margin of Gond-wana at about 45º southern latitude (Carls, 1988; Cocks and Tors-vik, 2002; Fig. 1D). Due to the general greenhouse conditionsduring the Early Devonian, the area was in a warm-temperateclimatic belt (Fig. 1D) with low latitude sea surface water tem-peratures ranging between 30–32 °C (Lochkovian) to 22 °C (lateEmsian; Joachimski et al., 2009).

The Early Devonian succession within the DARC is cha-racterized by a ca. 800 m-thick alternation of terrigenous clasticand carbonate deposits with abundant fossil remains (Fig. 2A).The predominance of carbonate- and fossil-rich sediments vs.terrigenous-clastic deposits has been related to variations in theterrigenous input from the CEM and CI, and the recorded car-bonate-clastic sequences (rythmothems) has been interpreted asformed in tune with deepening-shallowing episodes (Carls,1988, 1999). Within these successions, the ca. 140 m-thick No-gueras Fm (Lower Devonian: upper Lochkovian-lowermostPragian; Carls and Gandl, 1967) represents the first carbonate-dominated and fossil-rich sedimentary unit of the Lower Devo-nian (Fig. 2A). This unit locates between the dominantterrigenous-clastic Luesma and Santa Cruz formations. The No-gueras Fm is characterized by an alternation of dm- to m-thicklimestones and shales (mudstones) and occasional intercalationsof sandstones and marls, with abundant fossil remains (mainlybryozoans, brachiopods, crinoids, trilobites, tentaculitids, ce-phalopods, conodonts and vertebrates). The unit has been divi-ded in three members (d2a–d2c: Carls and Gandl, 1967), mainlybased on the predominance of limestones in members d2a andd2c, and shales in the intermediate member 2db (Fig. 2A).

The Nogueras Fm was interpreted as shallow marine depo-sits accumulated at depths less than 60 m (Carls, 1988, 1999).Most of the previous studies on this shallow marine unit havebeen focused on the general sedimentary context and paleonto-logy (Carls and Gandl, 1967; Carls, 1988; Carls and Valenzuela-Ríos, 2002), or in the analysis of specific fossil groups (e.g., fishremains: Botella and Valenzuela-Ríos, 2002; Botella et al., 2006,2009, 2012; brachiopods: Carls, 1974, 1985; Carls and Valen-zuela-Ríos, 1998; Carls et al., 1993; Schemm-Gregory, 2011;ostracods: Dojen, 2005; Dojen et al., 2004, 2007). However, todate, there is none integrated sedimentological and paleontolo-gical analysis that precisely defines the sedimentary model andthe biotic assemblages recorded in the different shallow marinedepositional subenvironments.

The main objective of the present work is the combined se-dimentological and paleontological analysis of the carbonate-rich lower member of the Nogueras Fm (upper Lochkovianmember d2a of Carls and Gandl, 1967; Fig. 2A) in the outcropsnear the village of Santa Cruz de Nogueras, located within theDARC, which correspond to the type area of the unit. Memberd2a has been selected for this study because: 1) it has a high va-riety of facies (carbonate, terrigenous-clastic and mixed facies),including the first fossil-rich carbonate deposits of the Devo-nian; 2) it crops out in stratigraphic continuity thus allowing adetailed bed-by-bed analysis and characterization of the mainfacies, fossil content and related depositional subenvironments.The overlying members d2b and d2c do not have the requiredconditions for a detailed bed-by-bed facies and paleontologicalanalyses due to their homogeneous lithology (shale-dominated

90 SEDIMENTOLOGY AND PALEONTOLOGY OF THE LOWERMOST DEVONIAN NOGUERAS FM


Fig. 1.- A, B. General geographic setting of the study area. C.Paleozoic outcrops of the Iberian Chains, indicating the locationof the Depresión Axial del Río Cámaras (DARC) of the HerreraUnit (adapted from Gozalo and Liñan, 1988). D. Global palaeo-geographic and paleoclimate reconstruction for the Early Devon-ian, and location of the Ibero-Armorican through (IAT; see insert),between the Cantabro-Ebroian Massif (CEM) and Central Iberia(CI). Arrows indicate detrital sources from the emerged areas.Adapted from data by Carls (1988,1999) on paleomaps by Blakey(2018) and Scotese (2018).

member d2b) or to the tectonic complexity of the outcrops(member d2c). The obtained results of the combined sedimen-tological and paleontological analysis will allow characterizingthe facies and main biotic assemblages within different shallowmarine subenvironments, and to discuss the possible signifi-cance of the alternation of carbonate-rich vs. terrigenous-richdeposits (i.e., clastic-carbonate sequences) in the context amixed clastic-carbonate system.

Geographic setting and methods

The outcrops of member d2a of the Nogueras Fm studiedhere are located in the surroundings of the village of Santa Cruzde Nogueras, in the northern part of the Teruel province, NESpain (Fig. 2B). The d2a member is ca. 40 m in thickness andhas been studied in two partial stratigraphic sections that haveallowed characterizing the entire d2a member: Las Viñas sec-tion (VI) and Virgen del Carmen section (VC), located respec-tively to the SE and NW of Santa Cruz de Nogueras (Fig. 2B).In the studied area, the Silurian-Devonian rocks are structured ina large recumbent synclinal fold (Fig. 2B). The section VI loca-tes in the southern part of the inverted limb (Figs. 2B and 3A),whereas section VC is placed in the northern part of the normallimb, near the hinge line in an abandoned galena mine and otheriron-rich minerals of hydrothermal origin (Figs. 2B and 3B).

The stratigraphic and sedimentological analysis has beenbased on a bed-by-bed and sub-bed level scale field study of li-thology, texture, components and sedimentary structures, whichwas complemented with the description of 31 rock samples inpolished slabs and 11 selected thin sections of the main faciesunder petrographic microscope. However, the limited lateralextent of outcropping beds (Fig. 3) has prevented the accurate

91M. Pérez-Pueyo, B. Bádenas and E. Villas


Fig. 2.- A. Synthetic stratigraphic log of the Lower Devonian of the Iberian Chains in the DARC (simplified from Carls and Valenzuela-Ríos, 2002). B. Geological map of the Santa Cruz de Nogueras area, with the location of the studied Las Viñas (VI) and Virgen del Car-men (VC) stratigraphic sections (adapted from Lendínez et al., 1989).

Fig. 3.- Field images of the lower member d2a of the NoguerasFm in VI section (A) and VC section (B). The black lines point outfaults and the red lines highlight the boundaries of the submem-bers defined by Carls and Gandl (1967): submember d2aα (in-cluding units 1 to 5) and submember d2aβ (including units 1 to 5).Notice the inverted bedding in VI (A) and the normal bedding inVC (B). In VC, the yellow stars indicate the bottom and the top ofthe stratigraphic section.

description of the lateral continuity of some sedimentarystructures (e.g., cross bedding and channelized bases).The purpose of this analysis has been the definition ofseveral sedimentological facies and the study of their ver-tical and lateral relationships. To unravel the most pro-bable lateral relationship of facies during deposition, theMarkov Chain stochastic model following the method ofHarms et al. (1982) has been applied to the observed ver-tical facies changes. This analysis consists on obtaininga matrix that evaluates the probability that one facies pas-ses to another, thus deciphering the most frequent faciestransitions.

A paleontological analysis has also been carried out bycharacterizing the skeletal debris in all beds and sampling en-tire fossils in 16 beds. The main fossil groups have been stu-died from a taxonomic, taphonomic and paleoecological pointof view. In addition, a calcareous bed bearing a great accu-mulation of fossils has been analyzed in detail in a square gridof 25x25 cm. Taxonomic counting and taphonomic analyses

have been performed in order to extrapolate the relative abun-dance of the main fossil groups and their preservation.

Results

Stratigraphic and sedimentological analysis

Lithological succession. Carls and Gandl (1967) iden-tify two submembers and different units within the lowermember d2a of the Nogueras Fm: submember d2aα (inclu-ding units d2aα1 to d2aα5) and submember d2aβ (includingunits d2aβ1 to d2aβ5). In the studied VI and VC sections(Fig. 3) these lithostratigraphic units have been identified(d2aα1–most of d2aα5 in VI; upper part of d2aα5–d2aβ inVC) allowing the correlation between both sections and thecharacterization of the entire d2a member. Nevertheless,there is some uncertainty concerning the thickness of unitd2aα5 due to tectonic deformation and the impossibility ofits physical tracing between both sections.



Table 1.- Main sedimentological features of the facies defined for the lower member d2a of the Nogueras Fm.

Mudstones(claystones/siltstones, C)

Sandstones (S)

Brachiopod marls (Bm)

Sandy limestones (Sl)

Packstones/Grainstonesof reworked brachiopod

(P/G brr)

Packstones/Grainstonesof ‘in situ’ brachiopods

(P/G bri)

Packstones/Grainstonesof reworked bryozoans

(P/G byr)

Packstones/Grainstonesof ‘in situ’ bryozoans

(P/G byi)

Packstones/Grainstonesof crinoids(P/G c)

Detrital mud

Detrital/calcareous mudWell sorted fine quartzgrains

Entire brachiopods (20-30%), usually articulated

Poorly sorted fine tocoarse quartz grains (50%)

Disarticulated valves androunded bioclasts ofbrachiopods (0.05-2 cm insize) (65%)

Entire brachiopods witharticulated valves (0.5-5cm in size) (80%)

Ferruginized, rounded andphosphatized bioclasts ofbranching andhemispherical bryozoans(0,5-8 cm) (50%)

Entire branching andhemispherical bryozoans,sometimes in life position(0,5-15 cm) (70%)

Bioclasts of crinoids(ossicles, and stemsfragments) (0,15-2 cm)(45%)

-

Bioclasts (<15%) of brachiopods, crinoids and bryozoans(fragmented). Occasional vertebrates(conodonts and fish remains?)

Entire bryozoans, orthocerids,tentaculitids, bivalves (10-15%)

- Bioclasts of brachiopods andbryozoans (30%) and crinoids (10%)- Occasional phosphate nodules andintraclasts

- Bioclasts of bryozoans, crinoids,bivalves, ostracods, trilobites (15%)and locally,fi¬sh remains - Iron ooids (5-10%)-Occasional phosphate nodules (mmto cm in diameter)

- Entire bryozoans and tentaculitids,bioclasts of crinoids (10%)

- Disarticulated valves and bioclastsof brachiopods (20-25%), bioclastsof crinoids (10%), and locally fi¬shremains. Occasionally phosphatized. - Local reworked phosphate nodules(mm to cm in diameter) and ironooids in vertebrate levels

Disarticulated valves and bioclastsof brachiopods (15-20%), andbioclasts of crinoids (10%)

Disarticulated valves and bioclastsof brachiopods (25-30%), andbioclasts of bryozoans (15%),ostracods and locally, ortocherids- Local phosphate nodules (mm tocm in diameter) and iron ooids

- Tabular cm- to dm-thick levels (30 cm in average) - Parallel lamination- Tabular or slightly wavy levels (10cm in average)- Cross lamination, current and waveripples and local bioturbation- Irregular to tabular cm-thick levels(8 cm in average)- Occasional bioturbation andintercalatedlimestone lenses

-Tabular dm-thick levels (25 cm inaverage), locally with channelizedbases (few cm-depth and few dm-long)-Planar, trough and herringbonecross-bedding, occasionalbioturbation and ferruginizations.

- Tabular dm-thick levels (30 cm inaverage), locally with channelizedbases (few cm-depth and few dm-long)- Occasional planar cross-bedding,ferruginized levels and bioturbation

Tabular cm- to dm-thick levels (10cm in average)

- Tabular to irregular dm-thick levels(15 cm in average) - Occasional planar cross-beddingand ferruginized crusts

- Tabular cm-thick levels (9 cm in average)- Bryozoans growing on ferruginouscrusts on tops of strata of P/G byrfacies

- Tabular dm- to m-thick levels (50cm in average)- Frequent parallel lamination withvariation in grain size, and frequentferruginous crusts- Local paleokarst surface

Carbonate

Mixed

Terrigenous-

clastic

FACIES MAIN GRAINS ACCESORY GRAINS BEDDING AND STRUCTURES



Fig. 4.- Las Viñas (VI) stratigraphic section of the lower d2a member of the Nogueras Fm. This section encompasses the submemberd2aα, from d2aα1 unit to the most of d2aα5 unit, defined by Carls and Gandl (1967).



Fig. 5.- Virgen del Carmen (VC) stratigraphic section of the lower d2a member of the Nogueras Fm. This section encompasses the upperpart of unit d2aα5 of the submember d2aα and the entire submember d2aβ, defined by Carls and Gandl (1967). See the upper part ofthe log in the next page.

The section VI starts at the base of the Nogueras Fm,which is marked by the first fossiliferous limestones on top ofthe dominant terrigenous-clastic succession of the LuesmaFm. This section is ca. 13 m in thickness and includes almostthe entire d2aα submember, from d2aα1 to most of d2aα5(Figs. 3A and 4). The succession is characterized by sandy li-mestones and skeletal limestones (d2aα1) passing upwards to

an alternation of mudstones, sand-stones and skeletal limestones(d2aα2–d2aα4), and to crinoidal li-mestones (d2aα5). The section VC isca. 24 m in thickness and encompas-ses the upper part of d2aα5 and thewhole d2a submember (Figs. 3B and5). This section includes in its lowerpart an alternation of skeletal limes-tones and fossil-rich marls (d2aα5 andd2aβ1–d2aβ2). Towards the top, ind2aβ3–d2aβ5, terrigenous-clastic li-thologies (mudstones, sandstones)dominate, and phosphate nodules andvertebrate remains occur.

Facies description. Field analysisand petrographic study of rock sam-ples in polished slabs and thin sec-tions have allowed to characterize 9facies, including terrigenous-clastic,mixed and carbonate facies, which arecomplexly intercalated at bed scale(Figs. 4 and 5). The detailed descrip-tion of facies is included in Table 1and the main sedimentary features areillustrated from field and sample ima-ges in figures 6 and 7.

Terrigenous-clastic facies en-compass mudstones (mainly clays-tones; facies C) and sandstones(facies S). Facies C is arranged incm- to dm-thick levels, has parallellamination and is barren of fossils.Facies S corresponds to cm-thickbedded fine-grained sandstones withlocal current and wave ripples, andfrequent parallel and cross lamina-tion and bioclasts of brachiopods,crinoids and bryozoans (Fig. 6A, B).

Mixed facies include brachiopodmarls (facies Bm) and sandy limes-tones (facies Sl). Facies Bm is arran-ged in cm-thick irregular beds andhas abundant articulate brachiopodshells and minor proportion of bryo-zoans, tentaculitids and orthocerids.The sandy limestones (facies Sl) arearranged in dm-thick tabular levels,with occasional low relief channeli-zed bases, and have planar-, trough-and herringbone cross-bedding cha-

racterized by sand-rich and skeletal-rich laminae with bio-clasts of brachiopods, crinoids and bryozoans (Fig. 6C, D).

Carbonate facies correspond to skeletal limestones, withpackstone/grainstone (P/G) texture. Based on the main ske-letal components (brachiopods, bryozoans and crinoids)and taphonomic criteria (reworked fossil debris vs. in situentire fossils), 5 carbonate facies have been differentiated



Fig. 5.- (Continued)

(Table 1). The P/G of reworked brachiopods (facies P/Gbrr) is arranged in irregular levels up to 30 cm-thick, withlocal low-relief channelized bases and planar cross-bed-ding, and is characterized by abundant disarticulated valvesand bioclasts of brachiopods, and minor proportion of bryo-zoans and crinoids (Fig. 7A, B). Iron ooids (with obscurednucleus and cortices due to complete ferruginization), fe-rruginous crusts on some bedding surfaces, and mm- to cm-sized phosphate nodules are also present. The P/G of in situbrachiopods (facies P/G bri) forms cm-thick tabular levelsplenty of well-preserved articulate brachiopod shells. It alsoincludes bioclasts of bryozoans and crinoids.

The P/G of reworked bryozoans (facies P/G byr) isarranged in cm-thick tabular levels with occasional tabular

cross-bedding and occasional ferruginous crusts on top.The facies bears abundant bioclasts of bryozoans, andminor proportion of brachiopods and crinoids and ironooids (Fig. 7C, D). The bryozoan debris are usually ferru-ginized, phosphatized and accumulated. Conversely, theP/G of in situ bryozoans (facies P/G byi) has well-preser-ved bryozoans in life position growing on ferruginous crusttop surfaces of facies P/G byr, as well as debris of brachio-pods and crinoids (Fig. 7E, F). Finally, the P/G of crinoids(facies P/G c) forms dm- to m-thick tabular levels and ismainly formed by crinoid ossicles and stem fragments anddebris of brachiopods and bryozoans. This facies also showcommon parallel lamination and ferruginous crusts, andlocal iron ooids and phosphate nodules (Fig. 7G, H).



Fig. 6.- A. Field image of facies C (mudstones), with intercalated sandstone facies S (sandstones). B. Thin section image of facies S(c: crinoid). C. Field image of Sl facies (sandy limestones), with a herringbone cross-bedding. D. Polished section image of Sl facies.



Fig. 7.- A, B. Polished section and thin section image of the limestones with reworked brachiopods, facies P/G brr (br: brachiopods).C, D. Field image and thin section image of the limestones with reworked bryozoans, facies P/G byr facies (br: brachiopods, by: bry-ozoans). E, F. Field image and thin section image of the limestones with in situ bryozoans, facies P/G byi facies (by: bryozoans; c:crinoids). G, H. Field image and thin section image of the crinoidal limestones, facies P/G c (br: brachiopods, by: bryozoans; c: crinoids).

Vertical facies stacking. Most of the described faciesare present and complexly intercalated along memberd2a, but some of them have a specific distribution. Thisis the case of the sandstone facies (S), which occurs ex-clusively in the lower d2aα submember (Fig. 4), and ofthe bioclastic marls (Bm) and the P/G of in situ brachio-pods (P/G bri) that are only present in the upper d2aβ sub-

member (Fig. 5). The results of the Markov Chain analy-sis applied to unravel the most probable vertical faciestransitions are summarized in Table 2. The facies transi-tions obtained are illustrated separately for submembersd2aα and d2aβ in Figure 8, as each submember includesa different suite of facies.

In submember d2aα (Fig. 8A), facies C (mudstones) andS (sandstones) are usually associated, being the sandstonesexclusively related (intercalated) with the mudstones. Theonly carbonate facies that is clearly related with the muds-tones is the P/G of reworked brachiopods (P/G brr), whichusually are seen in the field as thin beds intercalated withinthe mudstones. The mixed facies Sl (sandy limestones) isrelated exclusively with the P/G of reworked brachiopods(P/G brr). This P/G brr facies is also related with the bryo-zoan facies P/G byr and byi, which are intercalated betweenthem and with the P/G of crinoids (P/G c). Brachiopod andcrinoid facies hardly ever are related.

In submember d2aβ, similar facies relationships thanthose recorded in d2aα have been obtained for terrigenous-clastic facies (C and S), P/G of reworked brachiopods (P/Gbrr), bryozoan facies (P/G byr and byi) and P/G of crinoids(P/G c) (Fig. 8B). However, instead of the sandy limesto-nes (Sl), brachiopod marls (Bm) and P/G with in situ bra-chiopods (P/G bri) appear laterally related each other andwith the P/G of reworked brachiopods (P/G brr).

Sedimentary models. The sedimentary features of fa-cies and their vertical relationships (and lateral relation-ships during deposition, by the application of the Walther´slaw) allow the interpretation of their subenvironments ofdeposition. Due to the different set of facies recorded inthe two submembers d2aα and d2aβ (Fig. 8), two differentsedimentary models with subtle differences have been pro-posed (Fig. 9).

The sedimentary model for submember d2aα includes anintertidal area dominated by the deposition of terrigenous-

Facies

C

S

Bm

Sl

P/G c

P/G byr

P/G byi

P/G brr

P/G bri

Rj

C

-

0,665

-0,153

-0,201

-0,085

-0,182

-0,199

0,006

0,002

37

S

0,215

-

0,021

-0,092

-0,034

-0,102

-0,091

-0,099

-0,091

17

Bm

-0,110

-0,034

-

-0,092

-0,034

-0,022

-0,091

0,047

0,309

17

Sl

-0,052

-0,045

-0,046

-

0,017

- 0,048

-0,043

0,051

-0,043

8

P/G c

-0,062

-0,102

-0,044

0,027

-

0,052

0,237

-0,009

-0,096

18

P/G byr

-0,107

-0,142

-0,143

-0,011

0,170

-

0,199

0,050

-0,134

25

P/G byi

-0,039

-0,034

-0,034

-0,033

0,028

0,124

-

-0,030

-0,032

6

P/G brr

0,133

-0,273

0,310

0,429

-0,085

0,207

0,016

-

0,084

59

P/G bri

0,022

-0,028

0,089

-0,027

0,034

-0,030

-0,027

-0,038

-

5

Ri

37

16

17

8

16

25

6

62

5

192



Table 2.- Probabilistic matrix obtained after Markov Chain analysis, showing the probability of facies transitions. The most probablefacies transitions are outlined in light blue. Ri: number of transitions from a facies to other, Rj: number of transitions from any faciesto a concrete facies.

Fig. 8.- Vertical relationships between the described facies of thelower member d2a of the Nogueras Fm. Double arrows point outthe alternation between facies, whereas a simple arrow indicatestransition to top. The parenthesis marks occasional facies.

clastic sediments and a subtidal carbonate-dominated area(Fig. 9A). The terrigenous-clastic facies (C and S) representthe deposition of terrigenous muds in low energy conditions(C) and intercalated higher-energy coarser (sandy) sedimentswith ripples and cross-lamination (S). These facies fit wellwith the intertidal facies described in siliciclastic tidal flats,usually characterized by the alternation of muddy and sandysediments with small bedforms (e.g., Dalrymple, 2010).

The laterally related grain-supported carbonate facies(Fig. 8) are interpreted as subtidal sediments, including fromproximal P/G brr facies (P/G of reworked brachiopods), torelatively distal bryozoan facies (P/G byr and P/G byi) andcrinoid facies (P/G c), based on the deduced lateral facies rela-tionships. Their grain-supported texture, the predominance indifferent proportions of bioclastic debris of brachiopods,bryozoans and crinoids, indicate deposition in high-energyconditions. Planar-, trough- and herringbone cross-bedding,local low-relief channelized bases, and parallel lamination de-fined by variation in grain size, are indicative of the migrationof bedforms (megaripples/dunes) due to the action of tidaland/or wave currents within the shallow subtidal sub-environment. However, the limited lateral extent of outcrop-ping beds and the tectonic deformation have prevented a moreaccurate analysis of the lateral extent of cross bedding and ofthe required paleocurrent measurement to decipher the pre-

dominance of tidal or wave flows in theshallow area.

High-energy conditions are also in-dicated by the presence of iron ooids(Table 1). Devonian iron ooids havebeen also described in the CantabrianMountains (NW Spain) as original ironooids accumulated in high-energy sha-llow sublittoral environments, being thegenesis of iron compounds related to thesubaerial weathering of volcanic rocks(García-Ramos et al., 1987). In the stu-died case, the obliterated internal mi-crostructure of ooids has prevented toidentify the original composition ofooid lamina (carbonate or iron mine-rals), and thus to decipher the degree ofagitation based on the features of corti-ces of possible original carbonate ooids(e.g., Strasser, 1986) or the involved ge-netic processes of possible iron ooids.The higher percentage of ooids withinthe relatively shallower P/G brr facies(Table 1) points to they originate in sha-llow waters, rather that their generationin distal environments and resedimen-tation onshore related to condensed sec-tions (e.g., Collin et al., 2005).Ferruginous crusts are also usually pre-sent, more frequently in the shallowerP/G brr facies, and can be interpreted asgenerated by high-energy events thatwinnowed uncemented seabed sedi-ments and exposed the underlying ce-

mented substratum, rather than cemented surfaces generatedduring events of low sedimentation rates (Chris et al., 2012).Presence of phosphate nodules in these facies and their ge-neration linked with resedimentation processes in the sha-llow-water settings would be discussed in the next subsection.

The high-energy conditions in the shallow-water subtidalarea is also indicated by P/G of reworked brachiopods (P/Gbrr) that are intercalated as thin beds within the intertidalmudstones (facies C), which would represent bioclastic ac-cumulations in the intertidal zone, probably as the result ofstorm episodes. The sandy limestones (Sl), with planar-,trough- and herringbone cross-bedding and characterized bysand-rich and skeletal-rich laminae, were probably depositedin the intertidal-subtidal transition area, laterally related toboth S and P/G brr facies. Herringbone cross-bedding is wi-dely used as an indication of tidal deposition (Dalrymple,2010). In addition, alternations of siliciclastic and bioclasticforeset strata, quite similar to that recorded in facies Sl, havebeen described in subtidal tidal bundles in sand-dominatedtidal systems (Longhitano, 2011), but also in wave-domina-ted shallow water settings (Chiarella and Longhitano, 2012).The sandy limestones (SI) only appear in the basal part of thestudied Nogueras Fm and can be interpreted as a transitionalfacies between the Luesma and Nogueras formations. The fa-cies including in situ bryozoans growing on ferruginized sur-



Fig. 9.- Sedimentary models proposed for submembers d2aα and d2aβ of the d2a mem-ber of Nogueras Fm, showing the lateral relationships of facies. Question mark indicatesthe probable position of the hypothetic shoals protecting low-energy areas where bra-chiopod marls (facies Bm) were deposited. Sandy limestones (facies Sl) are related tohigh detrital input, whereas limestones with in situ bryozoans and brachiopods (faciesP/G byi and P/G bri) probably correspond to episodes of low sedimentation rates (alsocemented substrates for P/G byi).

faces (P/G byi) is very occasional and is interpreted as thelocal colonization of hardened subtidal P/G byr sediments.

In the sedimentary model for submember d2aβ, the bra-chiopod marls (Bm) would be in lateral relationship with thebrachiopod limestones (facies P/G brr and bri), as indicated bythe abundance of brachiopods in all these facies. There are twopossible subenvironments for these brachiopod-rich mixedmuddy sediments (Bm): 1) small ponds within the intertidaldomain; however, this is in disagreement with the MarkovChain analysis, as facies Bm is not related with the intertidal fa-cies C and S (Table 2 and Fig. 8); or 2) subtidal sediments lo-cated in protected - low energy- areas laterally related with thebrachiopod limestones (P/G brr and bri), an interpretation thatis coherent with the Markov Chain results. The existence ofprotected, low-energy subtidal areas would be due to possibleshoals developed seaward (Fig. 9B), although in the studiedoutcrops facies potentially representing high-energy barriers(e.g., facies with large-scale cross-bedding) have not been re-corded. Facies of in situ brachiopods (P/G bri) is an occasionalfacies that appears in the subtidal domain, in relationship withthe brachiopod marls (facies Bm) and P/G brt (Fig. 8B). Thesefacies bearing articulate brachiopod shells (Bm and P/G bri)are studied in detail in the paleontological analysis.

Sedimentary sequences. Considering the paleoenviron-mental interpretation of facies, at least 14 facies sequencescan be recognized in the lower member d2a of Nogueras Fm(Figs. 4 and 5). The sequences are very variable in thickness(around 4 m in average) and most of them are shallowing-up-

ward sequences (from dominant carbonate subtidal facies atthe lower part to terrigenous-clastic intertidal facies on top;Fig. 10A). The vertical arrangement of clastic and carbonatefacies in these sequences represents a ‘true’ mixed clastic-car-bonate system, instead of episodes of reciprocal sedimenta-tion controlled by external factors (e.g., see discussion inSchwarz et al., 2016). The upper boundary of each sequenceis a deepening surface, from shallower to relatively deeperfacies that can contain phosphate nodules. However, phos-phate nodules and phosphatized fossils are also presentthroughout the carbonate intervals of sequences, more fre-quently within the relatively shallower packstone/grainstoneof reworked brachiopods (facies P/G brr; Figs. 4, 5 and 10A).This points out that phosphatization was probably related todifferent stages of remobilization and remineralization of dis-solved phosphate derived from continental weathering in sha-llow water sediments, rather than phosphate-enrichment dueto coastal upwelling during possible transgressive episodes(e.g., Dornbos, 2011; Filipelli, 2011), proposed for the stu-died unit by Carls (1999).

The significance of the recorded shallowing sequences isout of the scope of this work and would require the analysisand correlation of sequences in separate sections. The studiedupper Lochkovian succession is around 40 m in thickness andencompasses an uncertain time interval of some Mys (the en-tire Lochkovian spans around 7.7 Mys: Da Silva et al., 2016).This data indicated reduced accumulation rates and subsi-dence, already suggested by Carls (1999). The recorded se-quences have an uncertain duration, within the range of



Fig. 10.- Comparison between (A) shallowing-upward sequence 11 (see location in left column in Fig. 5) and (B) a (deepening-sha-llowing) rythmothem reillustrated from Carls (1988). The rythmothem (without vertical scale in the original figure), include the follo-wing fossil groups: 1) turbid water (Rhenish) brachiopods, 2) clear water (Bohemian) brachiopods, 3) Fenestelid bryozoans, 4) clearwater trilobites, 5) turbid water trilobites, 6) pelagic fauna (cephalopods), 7) ostracods. Note that both in sequence 11 and the rythmo-them, carbonate sediments are located at the lower part, whereas terrigenous-clastic shallow-water facies dominate at the upper part. Ne-vertheless, these clastic deposits are low-energy (intertidal) facies in the sequence and high-energy facies in the rythmothem.

3th-order or 4th-order sequences of Vail et al. (1991) and theirorigin linked to tectonic pulses of accommodation gain or sea-level cycles is open to discussion. The presence of a serratedcm-depth irregular paleokarst surface on top of the crinoidalP/G in sequence 2 (Fig. 4), is a local evidence of subaerialerosion associated to a sea-level fall, thus indicating the pos-sible link of some sequences to low-amplitude eustatic sea-level cycles in the greenhouse Early Devonian epoch.

The sequences correspond to carbonate/terrigenous-clas-tic facies sequences similar to the rythmothems described byCarls (1988, 1999) for the Silurian and Devonian strata (Fig.10B); however, in the studied sequences neither the deeper,low-energy mudstones facies described within the rythmo-thems (interval 2 in Fig. 10B) nor the high-energy sandstonefacies (interval 3 in Fig. 10B) have been recorded. Instead ofshallow high-energy sandstone facies, intertidal mudstonesand fine sandstones (facies C and S) have been recognized.

Paleontological analysis

Description. The taxonomic and taphono-mic analysis of fossil groups has been carriedout with invertebrate fossils collected and iden-tified during fieldwork, especially those of Bmand P/G bri facies, and with invertebrate andvertebrate fossils observed in thin-sections.They were recognised and identified at least atOrder level, using as reference the work of He-rrera and Villas (2013) about the fossils of thestudied unit. Concerning invertebrate fossils,trepostomate bryozoans displaying branchingand hemispheric morphologies, crinoids, ten-taculitids and orthocerid nautiloid cephalopodshave been recognized. Brachiopods are themost abundant and diverse group and includethe following orders and genera: Orthida (Plat-yorhis, Isorthis, Schizophoria), Spiriferida(Howelella), Strophomenida, Terebratulida(Megantherys, Neopaulinella), Rhynchone-llida (Uncinulus) Athyridida and Atrypida. Bmfacies beds that have been sampled for fossils(Fig. 5) show the highest fossil diversity, withremains of all of the invertebrate groups des-cribed, except crinoids. In thin section fossilsof ostracods, trilobites and bivalves have beenobserved too. Vertebrate remains are scarce,and have been only identified in thin section.They correspond to conodonts and fish re-mains, specifically dental elements. In the area,several remains of conodonts and chon-drichthyes, thelodonti, acanthodian and placo-derm fishes have been described (Botella andValenzuela-Ríos, 2002; Botella et al., 2006,2009, 2012), so it is reasonable that the fossilobserved in the present work belong to one ofthese groups, but no further analysis has beencarried out.

The presence of a calcareous bed bearinga great accumulation of fossils, mostly bra-

chiopods (bed 46) belonging to in situ brachiopod limestones(subtidal facies P/G bri), has allowed a more detailed taxo-nomic and taphonomic analysis (Fi. 11). The large exposureif its top surface allowed a taxonomic counting using a gridof 25x25 cm to extrapolate the relative abundance of eachgroup in the whole bed. The results of this counting are re-flected in Figure 12. The bed is clearly dominated by orthidbrachiopods (62 %), including three main genera: Isorthis(Or1): with biconvex shells with fine ribs, Platyorthis (Or2),with plano-convex shells with fine ribs, and Schizophoria(Or3), with large dorsibiconvex and thicker ribs. The secondmain group of brachiopods are terebratulids (13%), all ofthem belonging to Megantherys genus. Other groups of bra-chiopods are athyrids (9%) and spiriferids (8%). Finally, bryo-zoans, atrypids and strophomenid brachiopods, crinoids andgastropods are also present in less proportion. In summary,bed 46 (subtidal facies P/G bri) has a high diversity of bra-chiopods, belonging to the main Devonian brachiopod groups(Clarkson, 1979).



Fig. 11.- Representative sample of bed 46 (see location in Fig. 5). At: athyrids, G:gastropods, Or1: Isorthis, Or2: Plathyorthis, Or3: Schizophoria, Spf: spiriferids.In this sample there are not any terebratulids.

The taphonomic analysis of brachiopod fossils in bed 46,included the study of features such as shell articulation/dislo-cation, size and shape selection, degree of fracturing, trend ofconcavity/convexity of valves and the amount of dorsal andventral valves preserved, which allow to obtain complemen-tary information about the depositional subenvironment offacies P/G bri. Taphonomic features have been also studied inother less abundant groups (Table 3). Thus, the fossil groupsof bed 46 have been divided, according to the terminology ofKidwell et al. (1986), in: 1) autochthonous/parautochtonousfossils (well-preserved fossils, with poor sorting, articulatedor little fractured shells, and sometimes in life position) thatmight live within or near the depositional subenvironment;and 2) allochthonous fossils (generally disarticulated andfragmented, well sorted and scarce), carried from their lifesubenvironment to a foreign one. Autochthonous/parautoch-

tonous fossils (90% of the fossils) in-clude orthid, terebratulid and athyridbrachiopods and bryozoans. Allochtho-nous fossils (10% of the fossils) encom-pass spiriferid and strophomenidbrachiopods and crinoids.

Interpretation. After the paleontolo-gical study complemented with the sedi-mentological analysis, some paleoeco-logical information on the main fossilgroups within the shallow marine depo-sitional environment can be inferred.Firstly, a clearly zonation can be deducedaccording to the facies distribution (Fig.8), with brachiopods living in the sha-llowest area of the subtidal domain, cri-noids dominating in the relatively deeperarea, whereas bryozoans would occupy

an intermediate position. Besides, fossil abundance and di-versity in the brachiopod marls (facies Bm) support the sedi-mentological interpretation of facies Bm as low-energy pro-tected areas in the platform (protected hypothetically byshoals) where brachiopods thrived on muddy sediments, ra-ther than ponds within the intertidal area. According to the fos-sil assemblages of Boucot (1975), intertidal areas are charac-terized by fluctuating low- and high energy conditions andscarce diversity of organisms, with only a few specialistforms, whereas in the subtidal areas the diversity increases, asis the case of facies Bm.

The taphonomic analysis of the in situ brachiopod li-mestones (subtidal facies P/G bri) in bed 46 displays a po-pulation of autochthonous-parautochthonous organisms(90% in total), and minor proportion of allochthonous or-ganisms (10%). The absence of bioclast lineations and the

predominance of oriented valves withconvexity oriented upwards (Table 3,orthids) point up that the shells whereaccumulated in an environment withcertain turbulence, but not submittedto a continuous agitation. So, we canconclude that the P/G bri facies repre-sents deposition in subtidal areas withpossible alternating low- and high-energy conditions and/variations in se-dimentation rates: episodes oflow-energy conditions and/or minorsedimentation rates favoured the sea-bed colonization by brachiopods andother organisms, whereas high-energyconditions would bury or slightly dragthe shell remains, and bring allochtho-nous skeletal remains from nearbyareas. It is reasonable to think that thisP/G bri facies, as well as the brachio-pod marls (facies Bm) were the sourcearea of skeletal remains recorded inthe limestones bearing transported bra-chiopods (facies P/G brt).



Group

Orthids

Terebratulids

Athyrids

Spiriferids

Strophomenids

Bryozoans

Crinoids

Taphonomic featuresPoor sorting (0.5-5 cm)Disarticulated shellsLow fracturation degree (35%)Dorsal/Ventral (51%/49%)Domination of convexity upwards (88%)Poor sorting (0.5-2 cm)Entire shellsLife position (33%)Poor sorting (0.8-1.5 cm)Generally entire shellsGood sorting (1-1.2 cm)Entire shells to fragmentsDisarticulated and fragmentedScarce presenceLife positionBrachiopods shells as substratumIsolated ossiclesScarce presence

Interpretation

Autochthonous-parautochthonous



Allochthonous

Allochthonous


Allochthonous

90% vs 10%% Autochthonous-parautochthonous vs allochthonous

Table 3.- Taphonomic features of the main fossil groups of bed 46, pointing out theirautochthony or allochthony.

Fig. 12.-Diagram showing the relative abundance of the different fossil groups in bed 46.

Finally, it is relevant to highlight that the organism co-llected and studied present adaptions for repelling coarse si-liciclastic sediment, as could be zig-zag commissures ofspiriferids and rhynchonellids (Uncinulus), or athyrid wrin-kles. The clearly absence of hermatipic corals (that usuallythrive in clear waters) also reinforces the interpretation of aturbid-water environment with suspended clays and silts thatwould reduce water transparency, similar to previous inter-pretations (Carls, 1988; Carls and Valenzuela-Ríos, 2002),identifying the fossils from the Nogueras Fm as Rhenishfauna, adapted to the described conditions.

Conclusions

Nine sedimentary facies have been recognized in the lowermember of the Nogueras Fm (d2a), according to lithology, tex-ture, main components and sedimentary structures: terrige-nous-clastic facies (mudstones and sandstones), mixed facies(brachiopod marls and sandy limestones) and carbonate facies(packstones/grainstones of transported and of in situ brachio-pods, packstones/grainstones of transported and of in situbryozoans and packstones/grainstones of crinoids). Based ontheir vertical distribution, two sedimentary models for sub-members d2aα and d2aβ have been proposed encompassingthe shallow areas of a mixed clastic-carbonate system.

The sedimentary model for submember d2aα shows twomain domains: a terrigenous-clastic intertidal zone (mudstonesand intercalated fine‐grained sandstones) and a carbonate-do-minated shallow subtidal zone with predominant high‐energyconditions (skeletal limestones with packstone/grainstone tex-ture). The main skeletal grains changes according to their po-sition in the subtidal zone: brachiopods dominate in shallowerareas, crinoids dominate in relative distal areas, and bryozoansoccupy an intermediate position. Mixed terrigenous-carbonatedeposits (sandy limestones) related with episodes of high de-trital input. The model for submember d2aβ includes similarintertidal sediments, but some specific facies in the subtidalzone, in particular brachiopod marly deposits. These marls arerelated to calm areas of the subtidal zone probably protectedby shoals. Phosphate nodules, phosphatized fossils, ferrugi-nous crusts and iron ooids, which are frequently associatedwith the relatively shallower bioclastic brachiopod facies, wereprobably linked continental sources and to remobilization inthe shallow water high-energy area. The defined facies arearranged in shallowing-upward sequences (from subtidal car-bonate-dominated sediments to intertidal clastic-dominateddeposits) of uncertain origin and duration (3th-order or 4th-order sequences; tectonic pulses or sea level cycles). They donot show the relative deep mudstone facies and the high-energy sandstone facies included in the rythmothems describedby Carls (1988, 1999) for the Silurian and Devonian strata.

The paleontological study has pointed the main groups oforganisms that inhabited this platform during the Early De-vonian. Fossil sampling has allowed identifying a great di-versity of organisms, predominantly brachiopods, associatedto local protected environments of the subtidal zone (bra-chiopod marls) and subtidal zones with alternating low- andhigh-energy conditions (limestones with in situ brachiopods).Taphonomic analysis also points a differentiation between au-

tochthonous/parautochtonous organisms (most of them bra-chiopods: orthids, terebratulids, athyrids and bryozoans) andallochthonous (spiriferids and crinoids) in the limestones within situ brachiopods. This study represents a preliminary pale-ontological analysis for future paleoecological studies des-cribing the communities associated to each of the definedfacies/subenvironments, where rich ecosystems of orthids, te-rebratulids, spiriferids and other groups developed.

Acknowledgements

This paper forms part of project H54 of the Governmentof Aragón (Grupos Consolidados and Dirección General dePatrimonio Cultural). We are grateful to Paul Wright and JuanCarlos Gutiérrez-Marco for their constructive comments onthe original version of the manuscript, to Zarela Herrera forproviding the image of Figure 11 and to the Associate Editor,Blanca Bauluz, for her editorial work.

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Revista de la Sociedad Geológica de España 31 (1)1)/RSGE31(1)_p_89_104.pdf · 2018-06-14 · Revista de la Sociedad Geológica de España 31 (1) ISSN (versión impresa): 0214-2708